This invention relates generally to a solid particle erosion resistant and oxidation resistant coating or a body being so resistant, wherein the coating or body utilizes titanium carbide particles and preferably angular titanium carbide particles, dispersed through a high chromium iron matrix material.
In steam turbine systems, steam can be contaminated by relatively small solid particles. It is believed that these solid particles are predominantly magnetite, Fe.sub.3 O.sub.4, and the particles may be referred to as "boiler scale" by persons of ordinary skill in the art. The solid particles are thought to originate in steam boiler tubing and adjoining pipes. During startup and normal operation of a steam turbine, these solid particles are carried through the entire steam turbine system by the steam flow. In this sense, the steam is called "contaminated" herein. Components within the steam flow path of the system are thus subject to solid particle erosion from solid particles impinging thereon, the extent of erosion depending in part upon velocity of the particles, steam pressure and temperature, and orientation of the component within the steam flow path, e.g. surface area of component normal to steam flow and angle at which particles impact the surface of the component.
Generally, turbine blades, which rotate within the steam turbine at high velocities, are especially subject to solid particle erosion, as are nozzle partitions, diaphragms, and areas within steam control valves. Valve stems and valve skirts are directly in the steam flow path and are greatly affected by solid particle erosion due in part to the sonic velocity which may be attained by steam passing through the valve during valving or throttling operations. Solid particle erosion may affect the useful life of steam turbine components. For example, some valves may have to be replaced or refurbished after an operating life much shorter than would be expected if the valve were operating in an environment free from solid particle erosion.
Titanium carbide has been well recognized in the metallurgical field as being very hard and resistant to many types of wear. Titanium carbide has been used with a steel matrix to provide a tool steel coating. The steel in the matrix of such a coatihg invariably has a martensitic metallurgical structure and can be applied, for example, by plasma spraying a relatively thin layer over the metal substrate. U.S. Pat. Nos. 3,896,244--Ellis et al., and 3,886,637--Ellis et al., disclose titanium carbide tool steel coatings. A processing technique disclosed by the '637 patent pre-alloys the coating material before spraying to ensure the presence of rounded grains of primary titanium carbide dispersed through a martensitic-containing steel matrix.
A coating or weldment including titanium carbide particles dispersed through a high chromium alloy matrix and deposited by plasma transferred arc application is disclosed by a brochure for TiCoat.TM. T-92 by Metallurgical Industries Inc., of Tinton Falls, N.J. The TiCoat.TM. T-92 coating is stated to be able to be deposited up to a thickness of 1/8" in a single pass and is recommended for use as a steel liner resistant to erosive wear by chilled cast iron grit utilized in a grit blasting operation. Another brochure by Metallurgical Industries on TiCoat.TM. T-93 states that the titanium carbide coating or weldment is applied by plasma transferred arc weld surfacing up to a thickness of 3/16" in a single pass. For multi-pass welding operations, titanium carbide will float or migrate to the upper surface of the previously deposited weldment, thereby resulting in a surface to which it is difficult to adhere another weld deposit and a loss in the uniformity of dispersion of titanium carbide throughout the coating. Attempts to increase the wettability of the previously deposited weldment include adding silicon and/or manganese. However, these additives promote the attainment of undesirable structures in the deposit, such as austenite, pearlite and sphereoidite.
Use of titanium carbide coatings disclosed by the prior art as an erosion resistant layer for steam turbine components, particularly valve components, is not acceptable in part due to demonstrated depth-of-coating limitations. Additionally, it is believed that a factor in determining the degree of erosion resistance of a titanium carbide layer is uniformity of dispersion of titanium carbide in a hard, erosion resistant matrix. It is also recognized in the art that because titanium carbide has a very low density with respect to typically employed matrix materials such as iron, nickel and cobalt, the carbide tends to float to the surface of a molten deposit such as is present in welding operations, thus destroying homogeneity and uniform carbide dispersion of the coating. Additionally, metallurgical analysis has shown that the matrix material that bonds titanium carbides together may erode preferentially with respect to the carbide particles, i.e. matrix material erodes before titanium carbide. Thus, solid particle erosion resistance of a coating including titanium carbide depends upon the process utilized to form the coating composite and to adhere titanium carbide particles and matrix material to the metal substrate, as well as the innate erosion resistance of the matrix which bonds the titanium carbide particles together and to the substrate. Of course, the deposition or consolidation process directly affects the degree of dispersement uniformity of titanium carbide in the coating. If the titanium carbide of the coating has been melted in the deposition process, it will be in the form of rounded grains and will also have a high likelihood of rising to the top of the deposit due to its reduced density. If the process of deposition or consolidation takes place in the solid state the carbide dispersion will be uniform and angularity or sharpness of the carbide particles will be retained.
Sintered powder metal products having titanium carbide additives are disclosed in U.S. Pat. No. 4,194,910--Mal. This patent discloses titanium carbide particles which are pre-alloyed by liquid phase sintering with iron and nickel base metals, and the resulting alloy is then dispersed through a base matrix, such as a steel matrix, wherein the matrix contains by weight 10% Cr, 2.9% Mo, 0.85% C and the balance essentially iron. Another U.S. Pat. No. 3,715,792--Prill et al., discloses a powder metallurgy sintered, allegedly wear resistant alloy having 45% TiC by volume in a high chromium alloy matrix. One of the disclosed matrices contains by weight 20% Cr, 0.8% C and the balance iron. In a specific example utilizing this composition, an alloy is produced which is annealed to generate a microstructure containing sphereoidite, and the alloy is thereafter hardened by heating to form a martensitic matrix. Other disclosures are to the effect that various titanium carbide alloys in hardened matrices resist erosion such as may be experienced in jet engine fuel pumps and valve seats. However, those disclosures do not specifically teach the chemical composition of the matrices or composition of the alloy, nor do they detail processing of the alloy and resulting metallurgical crystallographic structure of the alloy.
Accordingly, it is an object of the present invention to provide a solid particle erosion resistant coating or layer for steam turbine components.
It is another object of the present invention to provide a solid particle erosion resistant component body in the steam flow path of a steam turbine.
It is a further object of the present invention to provide a thicker solid particle erosion resistant coating comprising a more uniform dispersion of angular titanium carbide in a matrix alloy than is available using teachings of the prior art.
It is yet another object of the present invention to provide a method for applying to a substrate a solid particle erosion resistant coating using angular titanium carbide.
It is still another object of the present invention to provide a solid particle erosion and oxidation resistant coating containing titanium carbide and a matrix that itself is resistant to oxidation and erosion.
It is another object of the present invention to provide a method for applying to a substrate a solid particle erosion resistant coating wherein deposition and consolidation of the coating takes place in the solid state.